1. Field of the Invention
The invention relates to a hydropneumatic spring, which is preferably arranged between a vehicle body and a wheel guiding part in a motor vehicle, having a fluid-filled cylinder which is entered by an outwardly sealed piston rod connected to a piston which divides the cylinder into first and second working chambers, the first working chamber of the cylinder being in hydraulic connection with a first spring energy store via a valve device for altering the force-deflection progressivity characteristic of the spring (rate of change in the spring force as the spring is deflected), the second working chamber of the cylinder being in hydraulic connection with a second spring energy store, and a bypass connecting the first working chamber to the second working chamber when the hydropneumatic spring is in a mid deflection range.
2. Description of the Related Art
A prior art hydropneumatic spring having two spring energy stores is known from European reference EP 0 634 298 A1. In a spring cylinder, a piston rod is guided over a large portion in an axially movable manner and is sealed outwardly and with respect to a working chamber. The piston rod has a stop plate but does not have a piston. The portion of the piston rod with the stop plate enters the working chamber which is hydraulically connected to a spring energy store via a valve device. When the piston rod is in the mid-spring deflection range, an annular groove provided on the piston rod is connected to the working chamber via a bypass arranged in the piston rod, while a second spring energy store, between the seals, opens out into the piston rod guide. In the mid spring deflection range, the hydraulic connection between the two spring energy stores takes place via the annular groove and the bypass. Therefore, the force-deflection progressivity characteristic of the spring (rate of change of the spring force as a function of the amount of deflection) is consequently less or flatter in the mid spring deflection range. The complex structural design of this prior art spring cylinder entails high costs for the components and high costs for their assembly. In particular, in the spring range in which the second spring energy store is shut off from the annular groove, the pressure of the spring energy store has the effect of exerting a transverse force on the piston rod, which brings about an undesired increase in the frictional force during the axial movement of the piston rod.